Refactor drawable hierarchy to center properly

* Use FillFlowContainer.Spacing instead of manually applying margins.
* Use Update() for calculating button padding to preserve it after
  mod button expansion and adjust FooterButtonRandom to use this method
  while avoiding flickering.
* Expose mod display margin to clear it in the footer button.

osu.Game/Screens/Play/HUD/ModDisplay.cs

@@ -41,7 +41,7 @@ namespace osu.Game.Screens.Play.HUD
             }
         }
 
-        private readonly FillFlowContainer<ModIcon> iconsContainer;
+        protected readonly FillFlowContainer<ModIcon> IconsContainer;
         private readonly OsuSpriteText unrankedText;
 
         public ModDisplay()
@@ -50,7 +50,7 @@ namespace osu.Game.Screens.Play.HUD
 
             Children = new Drawable[]
             {
-                iconsContainer = new ReverseChildIDFillFlowContainer<ModIcon>
+                IconsContainer = new ReverseChildIDFillFlowContainer<ModIcon>
                 {
                     Anchor = Anchor.TopCentre,
                     Origin = Anchor.TopCentre,
@@ -69,11 +69,11 @@ namespace osu.Game.Screens.Play.HUD
 
             Current.ValueChanged += mods =>
             {
-                iconsContainer.Clear();
+                IconsContainer.Clear();
 
                 foreach (Mod mod in mods.NewValue)
                 {
-                    iconsContainer.Add(new ModIcon(mod) { Scale = new Vector2(0.6f) });
+                    IconsContainer.Add(new ModIcon(mod) { Scale = new Vector2(0.6f) });
                 }
 
                 if (IsLoaded)
@@ -92,7 +92,7 @@ namespace osu.Game.Screens.Play.HUD
             base.LoadComplete();
 
             appearTransform();
-            iconsContainer.FadeInFromZero(fade_duration, Easing.OutQuint);
+            IconsContainer.FadeInFromZero(fade_duration, Easing.OutQuint);
         }
 
         private void appearTransform()
@@ -104,17 +104,17 @@ namespace osu.Game.Screens.Play.HUD
 
             expand();
 
-            using (iconsContainer.BeginDelayedSequence(1200))
+            using (IconsContainer.BeginDelayedSequence(1200))
                 contract();
         }
 
         private void expand()
         {
             if (AllowExpand)
-                iconsContainer.TransformSpacingTo(new Vector2(5, 0), 500, Easing.OutQuint);
+                IconsContainer.TransformSpacingTo(new Vector2(5, 0), 500, Easing.OutQuint);
         }
 
-        private void contract() => iconsContainer.TransformSpacingTo(new Vector2(-25, 0), 500, Easing.OutQuint);
+        private void contract() => IconsContainer.TransformSpacingTo(new Vector2(-25, 0), 500, Easing.OutQuint);
 
         protected override bool OnHover(HoverEvent e)
         {

osu.Game/Screens/Select/FooterButton.cs

@@ -92,15 +92,17 @@ namespace osu.Game.Screens.Select
                             Origin = Anchor.CentreLeft,
                             Direction = FillDirection.Horizontal,
                             Shear = -SHEAR,
-                            AutoSizeAxes = Axes.Both,
+                            AutoSizeAxes = Axes.X,
-                            Margin = new MarginPadding { Horizontal = SHEAR_WIDTH / 4 },
+                            Height = 50,
+                            Margin = new MarginPadding { Right = -SHEAR_WIDTH / 2 },
+                            Spacing = new Vector2(15, 0),
                             Children = new Drawable[]
                             {
                                 TextContainer = new Container
                                 {
                                     Anchor = Anchor.Centre,
                                     Origin = Anchor.Centre,
-                                    Size = new Vector2(100 - SHEAR_WIDTH, 50),
+                                    AutoSizeAxes = Axes.Both,
                                     Child = SpriteText = new OsuSpriteText
                                     {
                                         Anchor = Anchor.Centre,
@@ -118,6 +120,19 @@ namespace osu.Game.Screens.Select
         public Action HoverLost;
         public Key? Hotkey;
 
+        protected override void UpdateAfterChildren()
+        {
+            base.UpdateAfterChildren();
+
+            float horizontalMargin = (100 - TextContainer.Width) / 2;
+            ButtonContentContainer.Padding = new MarginPadding
+            {
+                Left = horizontalMargin,
+                // right side margin offset to compensate for shear
+                Right = horizontalMargin - SHEAR_WIDTH / 2
+            };
+        }
+
         protected override bool OnHover(HoverEvent e)
         {
             Hovered?.Invoke();

osu.Game/Screens/Select/FooterButtonMods.cs

@@ -46,7 +46,6 @@ namespace osu.Game.Screens.Select
                 Anchor = Anchor.Centre,
                 Origin = Anchor.Centre,
                 Font = OsuFont.GetFont(weight: FontWeight.Bold),
-                Margin = new MarginPadding { Right = 10 }
             });
         }
 
@@ -94,6 +93,7 @@ namespace osu.Game.Screens.Select
             public FooterModDisplay()
             {
                 AllowExpand = false;
+                IconsContainer.Margin = new MarginPadding();
             }
         }
     }

osu.Game/Screens/Select/FooterButtonRandom.cs

@@ -24,8 +24,11 @@ namespace osu.Game.Screens.Select
                 Anchor = Anchor.Centre,
                 Origin = Anchor.Centre,
                 Text = @"rewind",
-                Alpha = 0
+                Alpha = 0,
             });
+
+            // force both text sprites to always be present to avoid width flickering while they're being swapped out
+            SpriteText.AlwaysPresent = secondaryText.AlwaysPresent = true;
         }
 
         [BackgroundDependencyLoader]